Polymorphism of (Z)-3-Bromopropenoic Acid: A High and Low Z′ Pair

Abstract

Two polymorphic forms of (Z)-3-bromopropenoic acid are reported. Form I (monoclinic, P21/c) with Z′ = 1 is obtained from a range of solvents, while Form II (monoclinic, P21/n) with Z′ = 4 can be prepared only from either benzene or toluene. Both forms are isolated at room temperature. The molecules in both polymorphs interact with one another through similar dominant hydrogen bonding motifs; however, the packing arrangement differs in the prevalence of weaker hydrogen bonds in the metastable Form II. Analysis of this high and low Z′ polymorphic pair using differential scanning calorimetry, grinding and slurry experiments, coupled with lattice energy calculations suggests that the low Z′ Form I is the most stable under ambient conditions. Two-dimensional fingerprint plots derived from Hirshfeld surfaces highlight the more extensive hydrogen bonding in Form II, while Form I is more densely packed. This polymorphic pair may be a candidate for the role of solution preaggregation in the formation of high Z′ forms.